Photopolymerizable compositions and films containing binder, monomer, initiator and chain transfer agent are described in the prior art and sold commercially. One important application of photopolymerizable layers is in graphic arts. Photopolymerizable layers are currently being used as electrostatic masters for analog color proofing and are considered as promising future materials to be developed for digital color proofing applications. For the analog color proofing application, a photopolymerizable layer is coated on an electrically conductive substrate to form an element and contact exposed with an ultraviolet (UV) source through a half tone color separation negative. The photopolymerizable layer hardens in the areas exposed with the UV source due to polymerization and remains in an unexposed liquid-like state elsewhere. The differences in viscosity between the exposed and unexposed areas are apparent in the transport properties, i.e., the unexposed photopolymerizable areas conduct electrostatic charge while the UV exposed areas are nonconductive. By subjecting the exposed photopolymerizable element to a corona discharge a latent electrostatic image is obtained consisting of electrostatic charge remaining only in the nonconducting or exposed areas of the layer. This latent image can then be developed by application of an electrostatic toner or developer to the surface. When the toner or developer has the opposite charge as the corona charge, the toner or developer selectively adheres to the exposed or polymerized areas of the photopolymerizable element.
Photohardenable electrostatic masters are needed that duplicate the imaging characteristics of a printing press. Such masters wherein the conductivity of both the exposed and unexposed areas can be controlled by introducing into a photopolymerizable composition an electron donor or an electron acceptor molecule that modify the electrical properties of the composition and provides the dot gain similar to that achieved by a printing press are known.
Although the use of photopolymers in electrophotography has been demonstrated and many formulations can be imaged, it did not appear possible, to produce a photopolymerizable or photohardenable electrostatic master that was capable of producing both positive and negative images. Photohardenable elements which have a conductive support bearing a photohardenable layer comprising a polymeric binder, a compound having at least one ethylenically unsaturated group, an initiator, a photoinhibitor and at least one sensitizing compound overcome the problem. These layers containing a photoinhibitor and a sensitizer compound are capable of producing positive and negative images depending on the exposure sequence and exposure wavelength. Such elements are extremely useful because a single element will satisfy the proofing needs of all printers regardless of whether they work with negative or positive color separations. A problem with these elements is that they require two exposures to provide a positive-working electrostatic master. Using the photosensitive element of this invention, which is not photohardenable, only one exposure is needed to provide a positive-working electrostatic master.
Several positive-working photosensitive electrostatic elements which also need only one exposure have been developed. In one such element the photosensitive layer has a composition consisting essentially of (A) at least one organic polymeric binder, (B) a hexaarylbiimidazole photooxidant, (C) a leuco dye oxidizable to an ionic species by the photooxidant, (D) a nonionic halogenated compound, and (E) a compatible plasticizer. While this photosensitive electrostatic master provides good results, it requires at least five components. The photosensitive layer of another such element generates sufficient acid directly on exposure to render the exposed areas conductive. While at least 1% by weight acid former must be used a preferred range of acid former is from 10 to 60% by weight. It has been found that using relatively less acid former can result in lower photospeed. It has subsequently been found that the ability of the unexposed element to hold charge at working humidities of 20% R.H. or more decreases fairly rapidly as the amount of onium salt acid former, e.g., triarylsulfonium hexafluoroantimonate salt, increases above 3% by weight based on the weight of the photosensitive layer. Also it is known that higher levels of onium salt initiators may not be completely compatible with the other components of the photosensitive elements.
It has now been found that in a photosensitive system containing an acid labile organic compound which decomposes to form acid only a catalytic amount of strong acid needs to be formed by an initiator on direct exposure. Charge decay occurs in the exposed image areas after a chemical amplification step occurs in which the catalytic acid decomposes the acid labile organic compound into sufficient free acid to render the exposed image areas conductive. This photosensitive system results not only in higher photospeed but significantly greater latitude in formulating the relatively nonconductive photosensitive layer, e.g., particularly when onium salts are used as the acid forming initiator.